In commercial distillation for the separation of one component or a plurality of components from mixtures containing the same, in many instances it is difficult to secure the desired degree of purity. A leading example of the difficulty of separating one component from another component is the breaking of an azeotrope. An azeotropic mixture is defined as a liquid mixture of two or more substances which act as a single substance in that the vapor produced by partial evaporation of the liquid has the same composition as the liquid. The separation of the substances becomes very difficult and expensive.
Azeotropic distillation has been used in the past to circumvent the problems caused when two substances form an azeotropic mixture. In this type of distillation, another substance is added to the azeotropic mixture in order to form another azeotropic mixture with one or more substances of the original mixture. The azeotrope or azeotropes thus formed will have boiling points which differ from the boiling points of the original mixture and will permit greater ease of separation. The problem with azeotropic distillation is that the addition of another substance to a mixture of other substances which need to be separated is not very efficient.
U.S. Pat. No. 4,906,787 (Huang et al.) discloses a process for producing diisopropyl ether containing negligible levels of contaminants, alcohol and water which process comprises hydrating propylene in the presence of an acidic zeolite. The result is an aqueous mixture of ether and alcohol. This aqueous mixture is then passed into a distillation unit operated at conditions which are effective to provide an azeotropic overhead fractionation comprising ether and minor amounts of alcohol and water. The azeotropic overhead fractionation is then passed to an alcohol separation unit which comprises an extraction column that uses process feedwater as the extraction medium. Huang et al. also teaches that the alcohol separation unit can be in the form of a decanter with the condensed azeotropic overheads separating into an ether-rich upper phase and an aqueous alcohol lower phase.
U.S. Pat. No. 4,345,973 (Ladisch et al.) discloses the recovery of ethanol from a fermentation broth comprising the steps of distilling a dilute aqueous alcohol to its azeotrope, distilling the azeotropic mixture using a third component consisting of either an organic solvent or a strong salt solution to break up the azeotrope and remove the remaining water, and distilling the resulting mixture to separate water from this third component.
U.S. Pat. No. 4,774,365 (Chen et al.) discloses a process for producing methyl tertiary butyl ether (MTBE). In the Chen et al. process, excess methanol and isobutylene are fed to an etherification reactor to produce an etherification reactor effluent comprising MTBE, methanol and isobutylene. This excess methanol tends to form azeotropes with the MTBE and isobutylene. To circumvent the azeotrope problem, an etherification effluent stream is passed to a pervaporation membrane to separate the methanol into a methanol-rich permeate vapor stream and an ether-rich raffinate stream. The methanol-rich permeate vapor stream is further cooled and compressed to recover the methanol. The ether-rich raffinate stream is sent to a distillation tower to separate the ether from the isobutylene.
U.S. Pat. No. 5,160,046 (Pasternak) discloses a membrane separation process. In the Pasternak process, an organic oxygen-containing liquid is contacted with a pervaporation membrane consisting of a polyimine polymer layer which has been interfacially crosslinked with a polyisocyanate linking agent to produce a permeate stream of decreased content of an organic oxygen-containing component and a retentate stream of increased content of an organic oxygen-containing component. Pasternak teaches that the process can be employed first and followed by distillation.
There is a need for an apparatus that combines both physical separation processes of distillation by fractionation and membrane separation by vapor permeation in a single vessel to be used in separating a process stream into a first component and a second component.